Method for asphalt compaction and compaction apparatus

ABSTRACT

A method of compacting a mat of hot mix asphalt which has been laid by an advancing asphalt paver, the method comprising advancing an asphalt compactor over the laid asphalt such that a compaction surface of the compactor, formed by a lower run of at least one belt, is engaged with any one portion of the mat for a period of at least 1.5 seconds, the compaction surface applying a maximum average load stress to the mat of less than about 50 kPa. Compaction may be achieved using a compactor comprising two longitudinally spaced modular compaction units connected relative to each other, and a power source for driving at least one of the modular compaction units, wherein at least one of the modular compaction units is adjustable to permit steering of the compactor, and wherein each of said modular compaction units comprises a compaction belt, support means for the belt to define a planar lower run of the belt forming a compaction surface.

This application is a 371 of PCT/AU97/00613, filed Sep. 18, 1997, now WO98/12386.

TECHNICAL FIELD

The present invention relates to a method for the compaction of asphaltand a compaction apparatus. More particularly, the present inventionrelates to a method and apparatus for compacting hot mix asphalt underconditions which advantageously optimise binder flow within the asphaltduring compaction.

By the term “binder” as used throughout this specification is meant anythermoplastic visco-elastic material which may be used in hot mixasphalts. Generally the binder will be bitumen or bituminous, that is abitumen incorporating, for example polymeric modifiers. It is also knownfor hot mix asphalt to incorporate polymer binders with no bitumen basedbinders present, and the present invention extends to the compaction ofall such hot mix asphalts.

BACKGROUND OF THE INVENTION

Inherent in modern asphalt mix design for heavy duty applications is theuse of components (aggregates and binders) which are purposely selectedto resist compaction and loss of shape under heavy traffic. Theseproperties will generally hinder the achievement of the desiredcompaction during laying of the asphalt.

The principal asphalt mix design element to resist compaction underheavy traffic is the use of aggregates with extremely rugose texture andcuboid shape, aimed at providing high shear resistance within theaggregate skeleton. In simple terms the objective is to ensure thephysical properties of the aggregate inhibit particle movement andpromote “lock up” in the structure under the applied load stress inoperation. Stiffer binders such as polymer modified binders areincreasingly being used to augment both the shear strength of the mixand also to improve the flexural or fatigue properties of the mix.

The achievement of lock up of the aggregate and the distribution of airvoids in the mix on compaction and during laying determines asphaltdurability and overall performance over the entire range of pavementloadings. Lock up of aggregate is advantageously achieved by displacingthe aggregate within the binder during compaction of the asphalt mat.

The properties of the asphalt mix are also determined by thevisco-elastic properties of its binder. At ambient service temperaturesthe binder desirably acts as a stiff elastic solid; the response to loadin the asphalt mix is very nearly elastic and a rapid load pulse willresult in a virtually instant elastic deformation which will recoveralmost the instant the load is removed. Thus, there is substantially noviscous flow and resultant permanent plastic strain. At the highertemperatures at which asphalt is laid and compacted, the binder in themix is a visco-elastic fluid. The higher the temperature, the lower theviscosity of the binder and the more readily the binder will deformunder any applied stress.

The compaction process begins with the laydown of hot asphalt by a paveron a prepared base, usually followed by pressure on the hot asphalt matapplied by a screed (with or without vibration). The screed is a plateor skid carried by the paver which slides over the surface of theasphalt mat desirably at or close to the temperature at which the mat islaid. The screed applies some initial compaction, but by its slidingaction may undesirably cause shear stress in the mat leading to tearingof the mat. Typically the applied static screed pressure is in the orderof 10 to 20 kPa and the load duration may be as long as 10-15 seconds.

Conventionally, asphalt compaction has been carried out using equipmentoriginally intended for compacting granular non-cohesive materialsdesigned to maximise the compaction energy applied to the material,primarily by using large and heavy steel drum rollers, often incombination with high energy oscillation or vibration. Rubber-tiredroller compaction is often used in conjunction with steel drum rollercompaction, as described hereinafter.

The contact stress between the roller and the asphalt mat generallydepends on the stiffness of the asphalt mix which is in turn stronglyinfluenced by the stiffness of the binder. The contact area between thesteel drum and the asphalt, that is the length of contact by the widthof the roller drum, will diminish as a result of the compactionachievement and the increase in mix stiffness with the cooling of themat. Typically the mix is at a temperature of about 150° C. when it islaid. In low temperature environments under adverse conditions such aswhen a strong wind is blowing, it is quite feasible the mix will cool tosay 140° C. at the bottom of the layer and 120° C. at the surface beforethe first compaction pass.

The largest dual steel drum vibratory roller compactor presently ingeneral use has a static mass of about 16 tonne with each drum having anaxial length of about 2 m. Assuming a nominal 100 mm contact length inthe roller direction (more in the initial pass, less in the final pass),each drum will apply a contact stress of about 400 kPa static andconsiderably more with vibration. In fact, each drum may apply a contactstress from about 100 kPa in a first static breakdown pass to well over1000 kPa as the asphalt mix stiffness and the contact area reduces.Compaction by the roller compactor usually occurs at varying distances,up to several hundred meters, behind the paver and at speeds of about1.1 m/s (4 km/h) or more. The two drums of the roller compactor eachhaving the above nominal contact length of 100 mm and therefore theroller will typically be in contact with any part of the asphalt mat forabout 0.2 seconds in each pass. Typically, about four steel rollerpasses are used, giving a total load time of about 0.8 seconds.

The roller compactor typically vibrates at about 20 Hz, which attemperatures of 140° C. and 120° C. corresponds to relatively highbinder stiffness (shown by Van der Poel's nomograph) of about 0.2 kPaand 1 kPa respectively (each 20° C. reduction in temperature has about a5 fold increase in bitumen stiffness).

As described above, the surface temperature of the mat may fall totemperatures of about 120° C. before the roller compaction process isbegun. The compaction process may typically include up to 4 rollercompactor passes, by which time the mat surface temperature may be inthe range 80-90° C. At mat temperatures below about 120° C. cracking ofthe mat may be initiated in the mat at high contact stresses,particularly at stresses induced using vibration. Mat cracking typicallyoccurs when the applied stress induces strain in the binder in excess ofits yield strength. At temperatures considerably above 120° C.conventional roller compaction may lead to significant shear failure inthe mat, depending on the asphalt mix type. This may result in the matbeing displaced laterally with loss of level and shape and ultimately inde-compaction of the mat.

Roller cracking resulting from low mat temperatures is usually manifestas fine, parallel cracks in the asphalt mat which are transverse to thedirection of rolling. A multi-wheeled rubber-tired roller following thevibratory roller compactor is commonly used to apply a kneading/shearingaction to at least the surface of the compacted asphalt mat, and therebycomplete the compaction of the mat. Such rubber-tired rolling is thoughtto close steel roller-induced cracks, at least at the surface of theasphalt mat, and increases surface texture by compressing the asphaltmortar between any coarse aggregate particles. Water is applied to thetires of the rubber-tired roller during rolling to alleviate materialpick-up. However, although the cracks may be closed at the surface thiswater may inadvertently be injected into the cracks before they aresealed, forming encapsulated water deposits beneath the surface of theasphalt mat. Encapsulated water may inhibit healing or encouragestripping in the asphalt mat.

U.S. Pat. Nos. 4,661,011 and 4,737,050 claim to alleviate roller-inducedcracking in the asphalt mat by use of an asphalt compaction machine inwhich pressure is applied to the asphalt mat through an endlesselastomeric belt extending between two rollers. The machine isconfigured to apply a more uniform pressure over the area of the belt incontact with the asphalt mat.

It has now been recognized in accordance with the present invention thatin a visco-elastic fluid, such as the binder in a hot mix asphalt, theresponse to load is not only temperature dependent but also timedependent. Thus, the application of a load of short duration will resultin an asphalt response which is more elastic than viscous as the bindersimply does not have time to flow. Therefore, using a vibratory rollercompactor at an accepted loading rate in the order of 20 Hz, the binderin the asphalt mix reacts during compaction more as an elastic solidthan as a viscous fluid and the compaction attempts to force theaggregate through the binder into a more compact arrangement, ratherthan causing the binder to flow around the aggregate with consequentmovement of the aggregate.

The previously mentioned Van der Poel nomograph provides an estimate ofthe stiffness of standard bitumen grades at selected rates of loadapplication and temperature. Even though the nomograph is well known tothose skilled in the art of asphalt compaction, the disadvantages ofapplying compaction loads of short duration have not previously beenfully recognized and short duration compaction using rollers with bothsteel and rubber interfaces, with or without vibration, has continued tobe the accepted practice.

It may now be recognized that by using the belt compactor of theaforementioned US Patents, improved compaction can be achieved byinducing viscous flow of the binder. Test uses of the belt compactor aredescribed, for example, by Halim OAE et al in “Improving the Propertiesof Asphalt Pavement Through the Use of AMIR Compactor: Laboratory andField Verification”, 7th International Conference on Asphalt Pavements,Nottingham, 1992. However, no recognition is given to the advantages oflonger load times.

The described belt compactor may apply a load stress of only about 5% ofthe aforementioned 16 tonne roller compactor under static load, butassuming conventional advancement rates are used the load may be appliedover a longer duration than a roller compactor due to the increasedcontact length of the belt. For a contact length of 1.25 m as describedin the aforementioned paper and a typical compaction speed of about 1.1m/s, the load duration will be about 1.1 secs. Using Van der Poel'snomograph, this increased load duration can be shown to reduce thebinder stiffness at 120° C. from about 1000 Pa for the aforementionedconventional vibrating roller compaction to about 5 Pa for the beltcompactor.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided amethod of compacting a mat of hot mix asphalt which has been laid by anadvancing asphalt paver, the method comprising advancing an asphaltcompactor over the laid asphalt such that a compaction surface of thecompactor, formed by a lower run of at least one belt, is engaged withany one portion of the mat for a period of at least 1.5 seconds, thecompaction surface applying a maximum average load stress to the mat ofless than about 50 kPa.

Without wishing to be bound by theory, it is believed that the presentinvention maximises the strength of the asphalt following compaction byemploying the visco-elastic behaviour of the binder during compaction,that is reducing the binder stiffness, allowing the binder time to flowaway from aggregate particle contacts while using the applied stress toreorientate the aggregate particles within the visco-elastic binder inorder to optimise intimate contact of the aggregate particles withoutthe application of high stress. On the other hand, the conventionalsteel roller compaction process described above focuses on the aggregatecomponents, using strong force to overcome the resistance to flow of thebinder and stress transfer from aggregate particle to particle toimprove the intimate contact between the particles.

The principal variables which can be used to reduce the stiffness of thebinder in the design asphalt mix are:

1. Asphalt Temperature:

using Van der Poel's nomograph, it is clear that increasing thetemperature of the asphalt at compaction by about 10° C. more thanhalves the binder stiffness; and

2. Load Duration:

again using Van der Poel's nomograph it may be seen that, for example, a10% increase in the duration that the compactor applies the load reducesthe binder stiffness by about 10%. Load duration may be varied bychanging either or both the length of the compaction surface and therate of displacement of the compactor over the mat.

In a first embodiment the method comprises advancing the asphaltcompactor over the laid asphalt substantially at the rate of advancementof the asphalt paver and within about 50 m behind the asphalt paver.

As may be readily seen from the above, the temperature of compaction isthe first key element in reducing the stiffness of the selected binder.Asphalt is generally manufactured at a temperature of about 160° C. andlaid at a temperature of about 150° C. By advancing the compactorimmediately behind the paver, that is with compaction being initiatedwithin about 50 m of the paver, in accordance with the above embodimentof the invention, the compaction method exploits the heat energysupplied in the asphalt manufacturing process.

By exploiting the low maximum average applied load stress with at leastsubstantially no shear stress, the method may advantageously beperformed at higher mat temperatures than conventionally used, forexample up to 160° C. Equally, the method of the invention may enablethe asphalt to be compacted at temperatures below the normal compactiontemperature. This may advantageously allow the asphalt to bemanufactured at a lower temperature than is conventionally used, withconsequential energy savings.

Advantageously, the compactor is advanced substantially at the rate ofthe paver within about 30 m, preferably within about 10 m, behind thepaver. In a preferred embodiment of the first aspect of the inventionthe asphalt compactor is advanced over the asphalt mat within about 5 mbehind the advancing asphalt paver and most preferably within about 2 mbehind the asphalt paver.

In this preferred embodiment, the compactor may be advanced by thepaver, that is the compactor may be connected to the paver. However,advantageously, the compactor belt is driven in order to minimise“shoving” of the asphalt being compacted. The drive is advantageously anauxiliary hydraulic drive. When the compactor is not connected to thepaver, the distance between the two, and therefore the speed anddirection of the compactor may advantageously be controlledautomatically via relative location sensor means.

As discussed above, a second key element in the compaction process isload duration. Assuming a typical asphalt placement rate of 1000 tonneper 6 hour day per paver, laying asphalt in a 50 mm thick layer, a pavermay travel at about 0.1 m/s. Higher paving rates, up to about 0.15 m/s,are known but not commonly adopted, and lower rates of 0.05 m/s or lessmay be used especially for thicker layers of asphalt.

Even advancing at the above maximum paving rate of about 0.15 m/s in themethod of the above embodiment of the invention, the compaction surfaceof the compactor belt is preferably engaged with any one portion of theasphalt mat for a period of at least about 7 seconds, ensuring a reducedbinder stiffness during compaction.

While the advantages of elevated temperature of the asphalt mat are bestachieved if the compactor follows immediately behind the asphalt paver,many advantages will still be achieved if the distance between the paverand compactor is increased. Particularly on small jobs, the rate ofadvancement of the compactor and therefore the distance of the compactorfrom the paver may be independent of the paver and still achieve the aimof the invention of reducing the binder stiffness during compaction byvirtue of a longer load duration than has been adopted conventionally.

Thus, according to a second embodiment of the invention the methodcomprises compacting the asphalt with the compactor by advancing thecompactor over the mat at a rate of no more than about 0.7 m/s.

By this embodiment of the present invention, taking the maximumdisplacement rate of about 0.7 m/s it will be understood that theminimum length of the compaction surface is about 1 m. This will resultin the compaction surface being engaged with any one portion of theasphalt mat for the minimum period of at least about 1.5 seconds, in anyone pass. This represents about a seven-fold increase over thetraditional roller compaction described above giving an even greaterreduction in binder stiffness at the same compaction temperature.

Preferably the total compaction duration in the method of eitherembodiment described above is in the range from about 7 seconds to about60 seconds, more preferably at least 10 seconds and most preferably atleast 15 seconds. This compaction duration may be achieved in a singlepass, although the load stress may be applied in two or more separatepasses by, for example two or more separate successive compactorsurfaces which closely follow one another. Preferably, the load isapplied in two or more separate passes, any one portion of the mat beingengaged by a compaction surface for a period of at least about 1.5seconds on each pass.

As noted above, the compaction duration may be varied by changing thespeed of compaction and/or the length of the compaction surface.Additionally, particularly in the method of the second embodiment of theinvention described above, the number of times the compactor isdisplaced over the mat surface may be varied. The rate of compaction inthe method of the second embodiment of the invention preferably is in arange from about 0.6 m/s to about 0.05 m/s or less, that is conventionalpaving speeds, more preferably from about 0.5 m/s to about 0.1 m/s.

The length of the compactor surface in either aspect of the invention ispreferably about 1 m, more preferably at least about 1.5 m, andoptionally may be about 2 to 3 m or more.

The maximum average applied load stress applied through the compactionsurface is preferably less than about 40 kPa, more preferably less thanabout 25 kPa. However, the applied load stress may increase graduallyfrom the leading edge of the compaction surface to the trailing edge, inwhich case the maximum line stress at the trailing edge of thecompaction surface is preferably about 40 kPa and the maximum averageapplied load stress is about 25 kPa. The minimum average applied loadstress is unlikely to be less than about 10 kPa. Such a low appliedstress would only be suitable for, for example, an asphalt mix to beused in residential streets in which a greater proportion ofvisco-elastic binder may be used and the degree of lock-up of aggregatenecessary for high traffic areas is not required.

Advantageously, as noted above the methods of the present invention maypermit the asphalt mat to be compacted to the desired degree in a singlepass, although variations in the compactability of the asphaltcomponents, the depth of the asphalt mat and the substrate temperaturemay require adjustment of the asphalt mix temperature and load durationfactors to achieve this. Correspondingly, the present invention maypermit deeper layers of asphalt to be laid and compacted.

The belt in the compactor used in accordance with this aspect of theinvention may be divided longitudinally to form two parallel tracks towhich varying drive may be applied to facilitate turning of thecompactor. With an elastomeric belt, different stresses may be appliedto opposite sides of the belt to facilitate turning. Alternatively, asingle belt compactor may be steered by the aforementioned connectionwith the paver or by a steerable tractor unit behind the compactor. Sucha tractor unit may be of a type well known for use with existingcompactors and may include track, tire or roller drive which may beadapted to provide additional compaction to and/or surface texture ofthe asphalt. Alternatively, again, the compactor may convenientlyinclude two longitudinally spaced belts, with the compactor being hingedbetween the belts to facilitate turning. By the method of the presentinvention the compaction surface of the belt may engage the mat surfacewithout substantial relative sliding movement in the displacementdirection therebetween because the or each belt rotates at thedisplacement rate of the compactor over the asphalt mat. It will beappreciated that there will be a small degree of relative slidingmovement at least partly in a lateral direction when the compactor isturned, but this degree of relative sliding movement will usually besufficiently small in use of the compactor as to not be substantiallydetrimental to the compaction of the asphalt. In preferred compactionprocedures in the method according to the second embodiment of theinvention, any turning of the compactor to reverse the direction ofcompaction is performed on previously compacted mat.

According to another aspect of the invention there is provided acompactor comprising two longitudinally spaced support assembliesconnected relative to each other, at least one of the support assembliesbeing adjustable to permit steering of the compactor, and a power sourcefor driving at least one of the support assemblies, and wherein at leastone of the support assemblies comprises a modular compaction unitincluding a compaction belt, support means for the belt to define aplanar lower run of the belt forming a compaction surface.

According to a further aspect of the invention there is provided acompactor comprising at least two longitudinally spaced modularcompaction units connected relative to each other and a power source fordriving at least one of the modular compaction units, wherein at leastone of the modular compaction units is adjustable to permit steering ofthe compactor, and wherein each of said modular compaction unitscomprises a compaction belt and support means for the belt to define aplanar lower run of the belt forming a compaction surface.

The compactor according to these aspects of the invention isparticularly suitable for use on a hot mix asphalt mat, but may also beuseful in the compaction of other paving materials.

Where only one of the support assemblies comprises a modular compactionunit, the other support assembly relative to which it is connected maybe, for example, an asphalt spreader in which case it may be used inaccordance with the method of the first aspect of the invention, or asteerable tractor unit in which case the compactor may be used inaccordance with either of the methods of the first and second aspects ofthe invention. In these embodiments, the modular compaction unit ispreferably, but not necessarily, pivotally connected by a hitch relativeto the other support assembly.

Alternatively, in accordance with the abovementioned aspect both of thesupport assemblies comprise modular compaction units each including acompaction belt, support means for the belt to define a planar lower runof the belt forming compaction surface. The units may be attached, forexample, by a hitch at one end of one unit pivotally connected relativeto the other unit. In this embodiment, the two modular compaction unitsare preferably pivoted relative to each other, for example by hydraulicmeans, to turn the compactor. In this arrangement, the two modularcompaction units advantageously replace two steel drum modules in anyknown articulated dual drum roller compactor.

Alternatively, again, the other support assembly may comprise, forexample, two belt compactors connected side-by-side, optionally in aspaced apart manner with the one modular compaction unit adapted tocompact the portion of the mat between the spaced belt compactors. Themodular compaction unit and the two spaced belt compactors may bepivoted relative to each other, for example by hydraulic means, to turnthe compactor. This arrangement may advantageously increase the width ofcompaction in a single pass.

It will be appreciated that when the compactor in accordance with thisaspect of the invention comprises a single modular compaction unit andthe aforementioned steerable tractor unit or two side-by-side beltcompactors, or two relatively pivoted modular compaction units, thecompactor is preferably, but need not be, used in accordance with themethod of either of the first and second aspects of the invention.

Preferably the modular compaction unit or at least one of the modularcompaction units is driven, that is rotation of its belt is powered.

Most advantageously, the or each modular compaction unit in a compactorin accordance with these aspects of the invention is designed to replacethe or each drum assembly in a conventional roller compactor.

The belt lower run in the or each modular compaction unit isadvantageously at least 1 m long, and may be as long as 2 or 3 m ormore. The belt in any aspect of the invention may be supported forrotation on the compactor by any suitable means. For example, in oneembodiment the belt extends between two or more drums or rollers, suchas two large diameter drums or a single larger diameter drum at theleading end of the compactor, which is preferably driven to alleviateshoving as described already, and two smaller drums or rollersrespectively defining the upper and lower runs of the belt at thetrailing end of the compactor. In another embodiment, the lower run ofthe belt extends between two relatively small drums or rollers and atleast one upper roller, which may be larger, supports the upper run ofthe belt. Between the leading and trailing ends of the lower run, thebelt may also be supported or engaged by any suitable means to providethe desired constant or gradually increasing load stress to the surface.For example, the aforementioned steel-segment belt may be supported byspaced rails or other guide means, while the aforementioned elastomericbelt may be supported by an array of intermediate rollers or drums or bya slide surface.

The width of the belt in the compactor used in the first aspect of theinvention is advantageously substantially the same as that of thespreader of the paver, for example 4 m, but may be less. For example,for smaller projects requiring manoeuvrability of the compactor it maybe convenient to have a smaller belt width such as approximately halfthe spreader width or less. Correspondingly, the belt width may in somecircumstances advantageously be smaller than that of the spreader, forexample 2 m or less.

The belt in any aspect of the invention may be formed of any suitablematerial taking into account the specific requirements of any particularapplication of the compactor. Thus, the belt may comprise elastomericmaterial such as laminated rubber, for example as described in theaforementioned US patent specifications. Alternatively, the belt maycomprise a series of pivotally interconnected rigid segments or, forexample, be formed of mesh or woven wire. Such segments, mesh or wiremay be formed of steel or other suitable material. Any suchnon-elastomeric belt may have elastomeric pads secured to the outersurface thereof to contact the material surface.

Using an elastomeric belt or a belt having elastomeric pads securedthereto on a hot mix asphalt will generally provide a better surfacetexture to the compacted asphalt than using a non-elastomeric belt alonedue to compression by the elastomeric material of bitumen around coarseaggregate fractions at the surface of the asphalt. However, when anon-elastomeric belt is used alone, a similar effect may be achieved bysubsequently rolling the surface with a rubber tired roller.

In order to alleviate heat loss from, for example, a hot mix asphaltduring compaction, except for its lower run the compactor belt in anyaspect is advantageously enclosed within the compactor. The enclosuremay be formed in part or wholly by an insulating shroud andadvantageously extends over the belt at least substantially to the levelof the compaction surface. Such a shroud may be formed in one or moreparts, for example from reinforced plastics such as fiberglass or ametal such as aluminium or steel with or without an insulating mat. Thebelt may be partly enclosed by a support system for the belt.

In some circumstances, particularly but not only in methods where thecompactor is not applied to the hot asphalt mat, it may be advantageousto heat the compactor belt. The compactor belt is preferably heated toat least the preferred temperature of the asphalt mat at compaction, forexample about 120° C. to about 150° C. or more, and may heat the asphaltmat during compaction. The heating of the compactor belt may also ensurethat the bitumen on the surface of the asphalt mat substantially doesnot adhere to the compactor belt.

The compactor belt may be heated by any suitable means, for example asuper-heated air generator or direct flame heating such as propane flameheating. Such heating means may be remote controlled, for example by ainfrared sensor aimed at the compactor.

BRIEF DESCRIPTION OF THE DRAWINGS

Alternatively, or in addition, the compactor advantageously includes oneor more reservoirs for hot liquid adjacent the belt. The hot liquid maybe, for example, heated oil or bitumen. The or each reservoir mayinclude means for heating the liquid therein as well as means forintroducing and draining the liquid from the reservoir.

A drum or roller associated with the compactor belt may act as areservoir for the hot liquid. Alternatively, or in addition, a separatehot liquid reservoir may be provided between two such drums or rollersor adjacent a single such drum or roller.

Various embodiments of methods and apparatus in accordance with one ormore of the aspects of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a side view of a paver and compacting apparatus working intandem and maintained at a constant separation distance via relativelocation sensors;

FIG. 2 is a plan view of the paver and compacting apparatus illustratedin FIG. 1 and clearly depicting the relative location sensors;

FIGS. 3 and 4 correspond to FIGS. 1 and 2 but show a modification inwhich the paving and compaction apparatus are physically interconnected;

FIG. 5 is a side view of compacting apparatus attached to a conventionaltractor from an articulated roller compactor;

FIG. 6 is a plan view of the compacting apparatus and tractorillustrated in FIG. 3; and

FIGS. 7 and 8 show, respectively, a side elevational view and a planview of self-powered compaction apparatus using two articulated modularcompaction units.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a compactor 10 compacts an asphalt mat 20which has been laid by a spreader 24 of a paver 22 on a previouslyprepared base 15. The compactor 10 is a belt compactor and followsimmediately behind the paver 22.

The compactor 10 includes a large diameter rotary drum 12 at a leadingend adjacent the paver 22, an upper transverse roller 14 a and a lowertransverse roller 14 b at a trailing end, and a hot liquid reservoir 13disposed between the rotary drum 12 and the rollers 14 a and 14 b. Thehot liquid reservoir 13 and the rotary drum 12 contain heated oil orbitumen at a temperature of about 150° C. The drum 12, rollers 14 a andb and the reservoir 13 are all supported by a framework 17 depictedschematically by a single frame member.

A laminated elastomeric belt 11 extends around the rotary drum 12 androllers 14 a and 14 b. The rotary drum 12 is driven by an auxiliaryhydraulic drive 19 and, therefore, imparts rotation to the belt 11 anddrive to the compactor. The belt 11, drum 12 and rollers 14 a and 14 bare split longitudinally with separate drives to the two halves of thedrum 12 to provide steerage to the compactor. The elastomeric belt mayadvantageously be replaced by, for example, a steel belt havingelastomeric pads secured thereto.

The lower run of the split belt 11 between the drum 12 and roller 14 bis supported against upwards deflection at the level of the commontangent of the drum 12 and roller 14 b by a slide surface defined by abottom wall of the reservoir 13. Preferably, but not shown, an array ofsmall rollers is provided beneath the reservoir 13 to support the beltin its planar lower run.

The compactor 10 also includes a thermal insulating shroud 16 whichclosely overlies the front, top and rear of the compactor and whichthereby alleviates heat loss from those portions of the belt not incontact with the surface of the asphalt mat 20. The shroud 16 may alsooverlie the sides of the compactor 10 and thereby further alleviate heatloss from the drum 12 and reservoir 13, and therefore also from theasphalt.

The compactor 10 travels at a distance of from about 1 to 2 m behind thepaver 22 at the speed of the paver. More particularly, the distancebetween an outer edge 23 of the spreader 24 for the asphalt and aleading edge 11 a of the lower run of the split belt 11 is from about 1to 2 m. The distance is maintained constant via relative location sensormeans 18 located at suitable positions on each side of the compactor 10and paver 22. The relative location sensor means 18 on each side maycomprise, for example, an infra-red or laser beam emitter supported onthe spreader 24 so as to emit the beam transversely to the direction ofadvancement, towards a target supported on a forwardly projectingelement 19 on the compactor 10. The target has a zero position and oneor more plus and minus positions on respective sides of the zeroposition. The preset speed of rotation of the respective drum 12 andbelt 11 is maintained while the beam hits the zero position of thetarget, but the speed will be temporarily increased or decreased if thebeam hits a plus or minus position, respectively. Such sensor means areknown but are advanced merely for illustrative purposes.

Typically, the paver 22 travels at a speed of about 0.1 m/s whilstlaying the asphalt mat 20. It will be recognized that the speed of thecompactor 10, therefore, will be substantially less than thatconventionally used in asphalt compaction processes. Furthermore, as thecompactor 10 follows immediately behind the paver 22, the temperature ofthe asphalt mat 20 is at or substantially at the spreading temperatureas compaction begins. The heating of the belt 11 by the hot liquid inthe drum 12 and reservoir 13, and the shroud 16, alleviate heat lossduring compaction, so that the temperature of compaction may be 150° C.or more.

As shown in FIGS. 1 and 2, the width Y of the compactor 10 and belt 11is 4 m and therefore such that the full width of the asphalt mat 20 laidby the spreader 24 is covered by the belt 11 on a single run of thecompactor 10. The length of contact X defined by the lower run of thebelt 11 is 3 m. For a compactor having a total mass of 24 tonne (240 kN)including the hot liquid in drum 12 and reservoir 13, a uniform contactstress of 20 kPa will be applied by the belt lower run. Assuming a speedof 0.1 m/s (typical for a placement rate of 1000 tonne per 6 hour dayper paver, laying asphalt in a 50 mm thick layer), the load duration atany point on the asphalt mat beneath the compactor belt will be about 30seconds. At this load duration and at 150° C., the binder stiffness willbe about 0.05 Pa.

The above size of compactor will be used in large scale projects. Insmaller scale projects the compactor 10 may have a much smaller“footprint”, for example a length of contact X of 2 m and width of 2 mor 4 m. A smaller footprint will generally correspond with a reducedmass of the compactor 10 as a whole. If so, this may be offset byincreasing the temperature of the process. In such a case, asteel-segment belt 11 may be used, heated by a direct flame.

Referring now to FIGS. 3 and 4, there is shown a modification to thecompactor 10 of FIGS. 1 and 2 by which the compactor 10 is physicallyinterconnected with the paver 22. The compactor 10 retains its ownauxiliary drive for the drum 12, so that the speed of advancement of thecompactor can be set to that of the paver. Thus, the mechanicalinterconnection between the paver and compactor is intended to provideonly steerage to the compactor.

The mechanical interconnection is shown schematically as the frame 26which projects forwardly from a leading end of the framework 17 of thecompactor to the sides of the spreader 24 and inwardly to a hitch 28beneath the paver. The hitch 28 may provide a rigid or pivotableinterconnection between the paver and compactor at the large radiuscurves confronted by the apparatus.

In operation, as the paver turns, this will be sensed by the frame 26which will mechanically impart the same turning motion to the compactor.A similar function may be achieved by replacing the frame 26 by, forexample, a simple cable arrangement.

FIG. 4 illustrates the longitudinal split of the compactor, includingthe drums, rollers and belt, and it will be appreciated that thecompactor may be made up of substantially identical modules, of forexample 1 m width, which are secured side-by-side to make up the desiredwidth of the compactor. If each of two belts in the compactor or eachouter belt has its own power supply, the speed of rotation of thesebelts may be adjusted individually to facilitate the turning of thecompactor. Any inner belt may not be powered.

FIGS. 5 and 6 better illustrate an alternative arrangement of thecompactor for use generally in smaller scale projects. In FIGS. 3 and 4,the compactor 30 has substantially the same set-up as the compactor 10shown in FIGS. 1 and 2 so will not be described in detail. The compactor30 includes the large diameter rotary drum 32 having an auxiliaryhydraulic drive, a hot liquid reservoir 34, the upper and lowertransverse rollers 36 and 38 respectively, a framework 40 supporting thedrum and rollers, a rotating belt 42 and a thermal insulation shroud 44.In this embodiment, however, rather than being maintained immediatelybehind the paver as in FIGS. 1 to 4, the compactor 30 is steered frombehind by a conventional tractor 46 from an articulated rollercompactor, the compactor being attached to the tractor by means of apivot connection 48 at one end of the framework 40. As before, the belt42 has a substantially rigid planar lower run but, for increasedmanoeuvrability, the lower run may have a reduced length of, forexample, 2 m or less.

A single belt 42, whether elastomeric or non-elastomeric, may be used inthis embodiment as steering is performed by the tractor 46 which haslarge diameter, liquid-filled smooth tires 50.

As with the compactor 10 of FIGS. 1 to 4, the hot liquid reservoirs 32and 34 may be enhanced or replaced by a super heated air blower ordirect flame heater for the belt. Such heating may be performedinternally of the belt, for example on the upper run, or externally, forexample between the shroud 44 and the drum 32 adjacent the lower run.Such heating of the belt may also be used to supply heat to the asphaltduring compaction, in which case satisfactory compaction with viscousflow of the binder may be achieved even though the asphalt has beenallowed to cool to a greater extent before compaction.

The compactor 30 includes an hydraulic jacking system 52 which isadapted to raise the belt 42 off the ground such that the belt is freeto rotate whilst the compactor is stationary. This facilitates evenheating of the belt prior to the start of a compaction run. The jackingsystem is carried by the framework 40 at the opposite end of thecompactor to the pivot connection 48 and incorporates a wheel assembly54 such that it may also be used to facilitate transportation andnon-use manoeuvrability.

The compactor 30 may be used at speeds up to about 0.7 m/s, which evenwith a belt lower run length of, for example, 2 m will provide acompaction duration of about 3 seconds in a single pass, substantiallymore than the described prior art. However, the compactor 30 willpreferably be used at speeds less than 0.7 m/s, for example about 0.5m/s or less, thereby increasing the load duration in a single pass. Thecompactor 30 may be used in the manner described with reference to thecompactor 10, that is immediately behind the paver and travellingsubstantially at the rate of the paver, but the compactor 30 will moreusually be used independently of the paver at the higher speeds. Underthese circumstances, the compactor 30 may readily have multiple passesover the asphalt mat to provide the desired degree of compaction. Eachpass may be between the paver and upto, for example, 400 m from thepaver, towards and away from the paver, and the speed of the compactormay be adjusted to enable the compactor to keep up with the rate ofpaving after the necessary number of passes. The compactor may apply auniform load stress of 20 kPa.

Referring now to FIGS. 7 and 8, there is shown a compactor 60 which isintended to be used in exactly the same manner as the compactor 30 ofFIGS. 5 and 6. However, the compactor 60 shows a modular form of beltcompaction unit, two of which replace the dual steel drums in a knownarticulated dual drum compactor. The known compactor comprises a powerand control module 64 and two drum modules which are partiallyillustrated by dashed lines 66 representing the drums.

Each compactor module 62 comprises a typical frame 68 having a hitch 70at one end for pivotal connection to the power and control module 64which sits between and above the compactor modules 62. The frame 68 inthe known drum compactor has the drum 66 journalled within the frame. Inplace of this, a smaller upper drum 72 for an elastomeric ornon-elastomeric belt 74 is journalled within the frame in the samemanner. Beneath the drum 72, the frame 68 supports a lower rollerassembly 76 for the belt. The roller assembly 76 comprises leading andtrailing rollers 78 and 80, respectively, of smaller diameter than thedrums 72, and an array of smaller intermediate rollers 82. The rollers78, 80 and 82 define a planar lower run of the belt which defines thecompaction surface of the compactor module 62. The lower run of the belt74 in each compactor module preferably has a length of 1.5 to 2 m, butmay be longer or shorter. As shown in FIG. 8, the belt width is about 2m to correspond with the standard drum modules, but may be more or less.

The drum 72 in each compaction module 62 is driven in the same manner asthe known drum 66 by the power and control module 64 through anauxiliary hydraulic drive (not shown). In addition to the connectiontogether of the compaction module 62 through the power and controlmodule 64, the compaction modules are connected by a steering hydraulicram 84, or preferably two steering hydraulic rams, one on each side ofthe hitches 70. The hydraulic ram or rams 84 are controlled by ahydraulic valve assembly (not shown) receiving steering inputs from thedriver of the compactor.

Each compaction module 62 has the belt 74 wholly enclosed except for thelower run beneath a shroud 86. The shroud helps to alleviate heat lossfrom the mat 88 during compaction, but advantageously also contain a hotenvironment for the belt. Such a hot environment may be provided by, forexample, providing hot liquid in the drum 72, but preferably is providedby super heated air supplied to the enclosure beneath the shroud by aheater on the compaction module or, more preferably, on the power andcontrol module 64. This heating of the belt helps to maintain a desiredcompaction temperature even though a particular portion of the mat 88may have cooled below that temperature by the time the compactor 60passes over it.

It will be noted in FIG. 7 that each compaction module 62 has asubstantially lower axes of rotation of the drum 72 than is the case forthe drum 66 in existing drum modules, leading to improved safetyparticularly on slopes.

It will also be appreciated that the compaction module 62 may readilyreplace the compactor 30 in FIGS. 5 and 6 as well as, with somemodification, the compactor 10 in FIGS. 1 to 4.

In each of the described embodiments, the belt compactor advantageouslyincludes means (not shown) for tensioning the belt. Such means mayinclude a roller or drum which is hydraulically displaceable.

It has been found that advantageously the asphalt compaction methods andcompactors according to the various aspects of the invention provideasphalt with significantly less permeability than asphalt compactedusing conventional equipment and techniques. In this regard, tests wereconducted in line with the New South Wales Road and Traffic Authority(RTA) Standard Test Method T168 (1990) entitled “Determination of InsituInfiltration of Water into a Road Pavement”. Briefly, according to thistest method a viewing tube provided with height markings is positionedsuch that it extends vertically above the area to be tested. The viewingtube is supported at is base by a base plate. Water is introduced intothe viewing tube and quickly brought to the desired height as marked onthe tube. The water then flows through the base plate and into contactwith the bitumen surface being tested. The rate of fall of the waterlevel between upper and lower marks on the viewing tube is recorded andthe porosity of the surface being tested calculated.

Using this method it was found that on testing asphalt prepared inaccordance with aspects of the invention, the time taken for the head ofwater to drop from 1 m. to 900 mm was in the order of 10 to 20 seconds.When conventionally compacted asphalt was tested on the trial site, theflow rate of water into the pavement was such that a head of water ofonly 200 to 300 mm could be maintained. It is believed that the higherpermeability of conventionally prepared asphalt surfaces may be due toroller cracking or non-closure of air voids and capillaries resultingfrom the conventional techniques.

Throughout this specification, unless the context requires otherwise,the word “comprise”, or variations such as “comprises” or “comprising”,will be understood to imply the inclusion of a stated integer or groupof integers but not the exclusion of any other integer or group ofintegers.

Those skilled in the art will appreciate that the invention describedherein is susceptible to variations and modifications other than thosespecifically described. It is to be understood that the inventionincludes all such variations and modifications which fall within itsspirit and scope. The invention also includes all of the steps orfeatures referred to or indicated in this specification, individually orcollectively, and any and all combinations of any two or more of saidsteps or features. For example, the invention may extend to a beltcompactor in which the belt is enclosed within the compactorsubstantially to the level of a lower run of the belt or to a beltcompactor in which means is provided for heating the belt, as described.Alternatively, the invention may extend to any other feature orcombination of features of the belt compactors described herein.

What is claimed is:
 1. A method of compacting a mat of hot mix asphaltwhich has been laid by an advancing asphalt paver, the method comprisingadvancing an asphalt compactor over the laid asphalt such that acompaction surface of the compactor, formed by a lower run of at leastone belt, is engaged with any one portion of the mat for a period of atleast 1.5 seconds, the compaction surface applying a maximum averageload stress to the mat of less than about 50 kPa, wherein the appliedload stress increases gradually from the leading edge of the compactionsurface to the trailing edge of the compaction surface.
 2. A methodaccording to claim 1, wherein the maximum line stress at the trailingedge of the compaction surface is about 40 kPa and the maximum averageapplied load stress is about 25 kPa.
 3. A compactor comprising at leasttwo longitudinally spaced modular compaction units connected relative toeach other and a power source for driving at least one of the modularcompaction units, wherein at least one of the modular compaction unitsis adjustable to permit steering of the compactor, and wherein each ofsaid modular compaction units comprises a compaction belt and supportmeans for the belt to define a planar lower run of the belt forming acompaction surface, and wherein in each modular compaction unit the beltextends between two large diameter drums or a single larger diameterdrum at the leading end of the respective compaction unit, which isoptionally driven, and two smaller drums or rollers respectivelydefining the upper and lower runs of the belt at the trailing end of therespective compaction unit.
 4. A compactor according to claim 3 whereinthe two modular compaction units are pivotally connected relative toeach other.
 5. A compactor according to claim 3 wherein the belt lowerrun in each of the modular compaction units is at least 1 m long.
 6. Acompactor according to claim 3 wherein in each modular compaction unitbetween the leading and trailing ends of the lower run the belt issupported or engaged to provide the desired constant or graduallyincreasing load stress to the surface of the material to be compacted.7. A compactor according to claim 3 wherein each of the belts compriseselastomeric material, a series of pivotally interconnected rigidsegments or is formed of mesh or woven wire.
 8. A compactor according toclaim 3 wherein each modular compaction unit is partially enclosedwithin the respective compaction unit, wherein the belt is enclosed inthe planar lower run of the belt.
 9. A compactor according to claim 8,wherein each belt is enclosed in part or wholly by a respectiveinsulating shroud which optionally extends over the belt substantiallyto the level of the compaction surface.
 10. A compactor according toclaim 8, wherein each belt is partly enclosed by a respective supportsystem for the belt.
 11. A compactor according to claim 3, comprisingheating means for heating each of the compactor belts.
 12. A compactoraccording to claim 3 wherein a respective drum or roller associated witheach compactor belt acts as a reservoir for hot liquid.
 13. A compactoraccording to claim 3 wherein a hot liquid reservoir is provided betweentwo drums or rollers associated with each of the compactor belts, oradjacent a single such drum or roller.